### Description Applying a simple `{s.prefixes}` transformation on a variable with an empty value crashes Kamailio. It is not specific to a particular SIP message being processed.

### Troubleshooting

#### Reproduction

``` request_route { $var(empty) = ""; $var(result) = $(var(empty){s.prefixes}); drop; } ```

#### Debugging Data

``` (paste your debugging data here) ```

#### Log Messages

``` kernel: [27707776.714159] kamailio[13060]: segfault at 562232762ff0 ip 00007f82a1e76391 sp 00007ffc3bc7fcd8 error 4 in libc-2.24.so[7f82a1d4d000+195000] ```

#### SIP Traffic

Any SIP traffic will do, as long as this particular scenario is hit, for whatever reason. Could be buggy business logic.

### Possible Solutions

The only thing I could think of is to place the transformation inside an `if` statement which checks for an empty value of the variable the `{s.prefixes]` transformation is being applied to.

### Additional Information

I've tested this with Kamailio 5.1 and 5.4, and both have this issue.

* **Kamailio Version** - output of `kamailio -v`

``` version: kamailio 5.4.0 (x86_64/linux) version: kamailio 5.1.10 (x86_64/linux) ```

* **Operating System**:

``` Kamailio 5.1.10 -> Debian GNU/Linux 9.13 (stretch) Kamailio 5.4.0 -> Debian GNU/Linux 9.8 (stretch) ```